1. Field of the Invention
The present invention relates to compositions for glass which are used for manufacturing fibers. In particular, the present invention relates to compositions which can be drawn in the form of discontinuous fibers with the aid of compressed air, water vapor jets, hot gas or centrifugal force.
2. Description of the Prior Art
Glass compositions currently employed in the manufacture of fibers have resulted from a compromise between the desired fiber properties, the costs of different starting materials and the constraints imposed by the process apparatus. In general, most of the known compositions employed for fiber manufacture satisfy some of the imposed constraints. However, such compositions suffer from other disadvantages such as those relating to economic considerations.
For example, many of the known compositions have a large number of constituents, which means that a large number of vitrifiable raw materials are needed to produce the glass. This multiplicity of raw materials limits the search for new sources of material supplies, and results in increased costs of stocking and storage.
Moreover, a number of the constituents of the known compositions come from raw materials which are, themselves, costly. The number and/or proportion of such constituents in the glass composition substantially increases the cost of the manufactured glass.
Further, the fusion of certain known compositions in gas-heated furnaces often results in the evolution of volatile materials. This volatilization necessitates an increased proportion of the more volatile constituents in the vitrifiable mixture, and requires installation of equipment to clean the offgases.
The relatively recent introduction of electric furnaces has led to a reduction in losses in volatilization. However, many of the known compositions, which are very satisfactory in some respects, are not suited to electric fusion. When such compositions are fused in an electric furnace, it is found that the layer of vitrifiable mixture which covers the glass batch is not stable. Gaseous pockets form which burst and intermittently expose the surface of the glass. These phenomena cause substantial heat losses and losses of material by volatilization which, and, while less important than the losses observed in gas heated furnaces, nonetheless pose a problem.
Therefore, a need clearly exists for a composition for manufacturing glass fibers which provides for the economical production of such fibers, and which can be fused stably and uniformly in an electric furnace.